1. Technical Field
The present disclosure relates to a dryer and an inkjet image forming apparatus.
2. Description of the Related Art
There is a growing need for small-lot multiproduct printing, for printing direct mails for individuals, etc., in recent years. Commercial offset printer is for large-lot printing and using a printing plate. Such an offset printer becomes more advantageous in cost performance and efficiency as the number of print copies increases. However, offset printer is unsuitable for variable printing such as small-lot multiproduct printing. For variable printing, on-demand printing using no plate is suitable. High-speed on-demand printers employing electrophotography is now spreading.
Another example of the on-demand printing includes inkjet printing. Since inkjet printing system is simpler than electrophotography, compact and budget personal inkjet printer is in widespread use. However, high-speed inkjet printer has not been actively developed in view of reliability of ink nozzle and printing speed.
Recently, the development of line head has advanced. Since line head does not require main scanning of ink nozzle, it is now possible to develop high-speed inkjet printer. Accordingly, there is a strong possibility that high-definition inkjet printer with a simple configuration is developed as on-demand high-speed printer.
Inkjet printer has some problems in a drying process. Low-speed printers for personal use have a problem of paper swelling caused due to moisture in ink. However, this problem is not fatal and can be solved by means of natural drying of the paper. With respect to high-speed printers, this problem cannot be solved by natural drying. When the printed copies are stacked, undesired phenomena such as offset, blocking, and color omission may occur.
Thus, the drying process cannot be eliminated from the inkjet printing process. As the drying process, drum drying that heats a drum, radiation drying that irradiates a target by a halogen lamp or infrared heater, and hot-air drying that blows hot air to a target have been employed. The drying process in inkjet printing corresponds to the fixing process in electrophotography. Therefore, the drying process damages one merit of inkjet technology, i.e., low energy consumption. Thus, it is required that the amount of energy consumed in the drying process is as small as possible.
The object to be dried is only ink. If other parts, such as paper or roller, are heated, energy is consumed unnecessarily. To selectively dry ink, means using frictional loss of dipole of dielectric body may be used, such as microwave and high-frequency wave dielectric heating. In this case, the calorific value depends on dielectric constant and loss tangent of the dielectric body. These values for water are extremely high. Accordingly, with respect to a medium on which an image is formed with an ink, the medium is not heated and only moisture in the ink is heated. Since only the amount of heat used for heating results in power loss in a high-frequency electric field, it is overwhelmingly advantageous in energy efficiency.
Microwave band is greater than high-frequency wave band in terms of loss tangent of water. Thus, microwave band is more advantageous for high-energy-density heating. However, there are some problems such as radio wave leakage and uneven heating. When a printer configured to successively take in/out a recording medium employs a dryer using microwave, the configuration may become complicated and the cost may increase. By contrast, high-frequency dielectric dryer is simpler in configuration, and has been widely used for print dryer.
Inkjet printing also has a problem of cockling. Cockling is a phenomenon in which paper having an ink image thereon swells by moisture in the ink and becomes undulate. In a case where a solid patch image is formed on paper, the solid image part is swollen by the ink but the peripheral non-image part is not. Cockling is caused due to a difference in the degree of swelling generated at an interface of the image. Actually, cockling starts growing upon impact of an ink droplet on paper, and the amount of cockling becomes maximum several tens of seconds later. The order of the amount of cockling corresponds to the time scale of permeation and swelling of paper fiber. The amount of cockling thereafter decreases by natural drying, however, does not become zero. This is because strain, which has been generated due to swelling of paper, is still remaining. With respect to high-quality printing such as offset printing, even a slight amount of cockling may degrade the image quality. Accordingly, how to suppress the occurrence of cockling is one object for inkjet printing that is one of high-quality printing technologies.
One additional problem is caused when a conductive-particle-containing ink, such as a black ink containing carbon black particles, is used. As conductive particles for black inks, carbon black is superior in terms of density, texture, and color development and is widely used. An ink dispersing carbon black particles exhibits no conductivity. As carbon black particles in a black solid image come into contact with each other as drying of the image progresses, the image exhibits conductivity in a direction of the plane thereof. High-frequency dielectric heater, which is one type of heaters heating a dielectric body, may cause dielectric heating or spark, if a conductive body exists, depending on the resistance value of the conductive body.
Thus, when a solid image formed with a black ink containing carbon black is heated by a high-frequency dielectric heater, abnormal heating may occur and the image may get burned as drying of the image progresses. In this case, since the black image gets burned before other inks containing no conductive particle are completely dried, the drying process cannot be conducted at all.